This invention relates to methods for combusting carbonaceous fuels including gaseous fuels such as natural gas and liquid fuels such as fuel oil. In U.S. Pat. No. 3,928,961, granted Dec. 30, 1975, in the name of William C. Pfefferle and assigned to the same assignee as that of the present invention, there is disclosed and claimed a process designated catalytically-supported, thermal combustion. According to this method carbonaceous fuels can be combusted very efficiently and at reaction rates characteristic of thermal (homogeneous) combustion, although the combustion is effected in the presence of a solid oxidation catalyst at temperatures below nitrogen-oxide-forming temperatures. Ordinarily the operating temperature of a catalyst in catalytically-supported, thermal combustion is in the range of approximately 1700.degree.-3200.degree. F., for example near the middle of this temperature range. It has been found to be desirable, for combusting various types of carbonaceous fuels, to carry out the combustion in more than one stage, including at least one stage utilizing a catalyst and at least one thermal combustion stage. Thus, in the aforementioned U.S. Pat. No. 3,928,961 there is described and claimed the method in which catalytically-supported, thermal combustion is carried out in the presence of a catalyst, followed by thermally combusting a partially combusted effluent from the catalyst stage. Also, in another U.S. patent in the name of William C. Pfefferle, U.S. Pat. No. 3,846,979, granted Nov. 12, 1974, to the same assignee as that of the present invention, there is disclosed a method in which carbonaceous fuel is partially combusted in a thermal combustion zone, followed by immediately quenching the effluent containing the partially combusted fuel, and then contacting the quenched effluent with a catalyst to oxidize some or all of the uncombusted fuel from the thermal combustion zone.
In one of the examples in the last mentioned U.S. Pat. No. 3,846,979, diesel fuel and compressed air in an amount 10% in excess of the stoichiometric amount of air for complete combustion of the fuel are supplied to the thermal combustor to produce an effluent of about 90% oxidized fuel, which is quenched with sufficient excess air to provide an adiabatic flame temperature of 1500.degree. F. (815.degree. C.). The fuels mentioned in the examples of that patent are diesel fuels, or jet fuels of the type customarily used in aircraft gas turbine engines. Under typical conditions for such engines, operating at a pressure of 20 atmospheres, the quenched effluent is said to have an adiabatic flame temperature of 1500.degree. F. The actual temperature of the quenched effluent would be about 100.degree.-200.degree. F. below its adiabatic flame temperature, i.e., above 1300.degree. F. (above 700.degree. C.), by virtue of the small portion of unoxidized fuel from the thermal combustor. According to this example in the patent sufficient additional fuel is sprayed into the effluent to raise the adiabatic flame temperature to 2600.degree. F. (1425.degree. C.), and the resulting admixture then is passed to the catalyst stage and fully combusted. The temperature at the outlet of the thermal combustion zone is said to be in excess of 3300.degree. F. (1800.degree. C.) before quenching and the outlet temperature from the catalyst is 2500.degree. F. (1370.degree. C.).
While the combustion method just described is suitable for certain types of fuels and operating conditions, the quenching operation tends to be difficult to carry out with the amounts of fuel provided in the mixture entering the thermal combustor. Thus, vigorous mixing of the quenching air with the partially oxidized thermal combustion effluent is required to stop the oxidation; this mixing involves turbulence with undesirable pressure drops in the quenching zone, and the combustion-quenching action still may be difficult to sustain in the event of transient disturbances. Moreover, reliable avoidance of preignition and further combustion, as soon as the additional fuel is sprayed into the quenched effluent as disclosed in the patent, may be difficult in practice unless the quenched effluent is substantially cooler than that indicated in the example of the patent. It is an object of the present invention to overcome such problems in carrying out a two-stage combustion process utilizing a thermal combustion stage followed by combustion in the presence of a catalyst.
Another invention of William C. Pfefferle, also assigned to the same assignee as that of the present invention, is described in U.S. patent application Ser. No. 644,873, filed Dec. 29, 1975, now U.S. Pat. No. 4,019,316. That invention addresses the problem of starting a combustion system utilizing a catalyst. A fuel-air mixture is combusted at start-up in a thermal combustion zone to provide a source of heat, and this heat is directed to the catalyst in the substantial absence of unburned fuel to bring the catalyst to a temperature at which it will sustain mass transfer-limited operation. The catalyst temperature attained during this start-up operation thus would be sufficient to ignite a mixture of carbonaceous fuel and air, and such catalyst temperature may approach the operating temperature for effecting catalytically-supported, thermal combustion of such mixture in the presence of a catalyst. The purpose of the invention of the aforesaid application Ser. No. 644,873, now U.S. Pat. No. 4,019,316 is to avoid the grossly incomplete combustion which may occur in the catalyst if a fuel-air mixture passes through the catalyst while it is cold. The effluent from such incomplete combustion may pollute the surrounding atmosphere with undesirably high amounts of unburned hydrocarbons and carbon monoxide. Effluent from the thermal combustion zone utilized during start-up in accordance with the invention of the application Ser. No. 644,873, now U.S. Pat. No. 4,019,316 is not permitted to contact the catalyst unless it is free of unburned fuel, which has the dual advantage of avoiding damage to the catalyst through burning of fuel after absorption on the catalyst when cold, or, in the absence of such burning on the catalyst, of avoiding passage of unburned fuel through the catalyst to the atmosphere. As soon as the catalyst has been heated to a temperature at which it can function to burn the fuel in the mixture passing through it, practically all of the fuel-air mixture is fed directly to the catalyst for sustained, steady state operation, and the fuel supply to the thermal combustion stage used for start-up is cut off or cut back drastically. Thus after ignition is achieved in the catalyst, the thermal combustion either ceases, or is carried out at a low level of fuel supply at which it serves an entirely different function and merely assists in vaporizing the fuel. There may be an intervening short period during which the thermal preburning continues at appreciable levels until regenerative preheating of the inlet air is established, this period depending on the initial temperature and the mass of the heat exchanger.
While the above described method of application Ser. No. 644,873, now U.S. Pat. No. 4,019,316 may be eminently useful in achieving start-up in a combustion system utilizing a catalyst, it is an object of the present invention to effect sustained combustion of carbonaceous fuel in a combustion system utilizing a catalyst wherein a preburner is used throughout the operation of a combustion system, or during sustained combustion therein under certain operating conditions such as operation within predetermined fuel demand ranges or operation utilizing predetermined fuel-air ratios.